Comet Hale-Bopp Ion Tail May Be Disrupted

NASA/ESA scientists say that between 28 April and 15 May, comet Hale-Bopp
and especially its blue "ion" tail will be most susceptible to influences
from the Sun's solar wind and magnetic field. Its ion tail is expected to
begin showing fluctuations, kinks, and perhaps moving structures. Also, it
is possible that a disconnection event might occur, when the ion tail breaks
off from the comet's head and reforms a few hours later.

The Sun has a permanent abrupt and well-known change in magnetic field
direction in the solar wind located close to the solar equator, which the
comet will intersect. The Sun also releases a large amount of ionized gases
from quick-acting disturbances, like that recently occurring on April 7.
Both types of solar phenomenon can dramatically alter the comet's tail and,
on rare occasions, can "disconnect" the comet ion tail from the comet head.
Comet Hale-Bopp has two tails, from the two types of material evaporating off
the "dirty snowball" nucleus as it warms in the sunlight. There is a curved
dust tail of fine gritty hard material that brightly reflects sunlight and is
easily observable. The other, dimmer tail is an ion tail, distinguished by its
bluish color in photographs. This tail consists of gases of water, carbon
monoxide and other simple atoms and molecules evaporated off the comet that
have become electrically charged by interacting with sunlight. The charged
atoms and molecules, called ions, are then pushed along by the interplanetary
magnetic field embedded in the solar wind. (The solar wind is a stream of
charged particles that comes from the sun and drags the solar magnetic field
out into interplanetary space). It flows out from the Sun at tremendous speed
(about a million miles per hour), more than a hundred times as fast as the
comet.

Thus, the comet ion tail responds as a sort of "wind sock", that gets de-
flected in the direction of the outflowing solar wind. If the interplanetary
magnetic field and solar wind are disturbed, the effect will become quite
visible, most likely as a rippling in the ion tail.

Hale-Bopp is now rapidly approaching the solar equatorial region, where the
wind is gusty and the magnetic field irregular. There it will encounter
well-known changes in the main direction of the interplanetary magnetic field.
On April 21, Hale-Bopp will be down to 15 degrees from the solar equator, and
on May 3, it passes through the plane of the solar equator into the southern
solar hemisphere.

The region of interplanetary magnetic field reversal is shaped like a wavy
disk which rotates with the sun once in 25 days. The complex solar wind is
now confined to about 25 degrees from this wavy disk. Also, the tilt of the
wavy disk is itself less than 15 degrees. A much more precise estimate can
be made with a current sheet map, based on measurements from spacecraft, including WIND, SOHO (Solar Heliospheric Observatory), and Ulysses. (The
first two are part of NASA/ESA's International Solar Terrestrial Program
(ISTP) which analyzes data in correlative studies of the Sun and solar wind.)
The map shows that the region of complex wind will be sinking to the south
as Hale-Bopp is approaching 15 degrees, so that this comet's ion tail will
not likely be disturbed until it gets down to less than 10 degrees, soon
after April 28. The first half of May should be the optimal time of dis-
ruptions in Hale-Bopp's ion tail. Scientists anticipate that the major
characteristics of the map of solar wind will remain stable, but they also
say that the Sun could eject an unexpected quick-acting disturbance at any
time, similar to one on April 7.

Comet observers all over the world will be looking at Hale-Bopp for disruptions
all through this period. A person viewing with the naked eye might
see a "lumpy object" or luminous mass in the comet tail, this traveling
outward along the tail for a number of days. With photographic equipment,
this mass should be resolvable as an anomaly in the ion tail. Solar astron-
omers are also watching for any solar activity on the east solar limb which
might cause additional disruptions.